The invention relates to an apparatus and method for generating tone signals useful in telephone systems.
In telephone systems various machine generated voice frequency signals are used for transmitting supervisory and signalling information between switching facilities and for transmitting progress signals to telephone users. In a central office switching facility (CO) multifrequency (MF) signals are typically used for inter CO trunk signalling. In a private branch switching facility (PBX) duel tone multifrequency (DTMF) signals may be used to transmit dialled digits to a central office switching facility (CO). The DTMF signal family is distinctively different in comparison with the family of MF signals in order to better avoid any operational confusion between the two. Hence it has been traditional practice for a telephone equipment manufacturer to produce at least two types of tone signal generators, one for the CO environment and a different one for the PBX environment. Additional types of tone signal generators are also typically produced for providing call progress tones, for example dial tone, busy tone etc., and for providing transmission facility test tones. If the manufacturer supplies market places involving two or more telephone operating standards, for example in North America and in Europe, the number of different types of required tone signal generators likewise increases. This is particularly so in time division multiplex (TDM) telephone systems wherein pulse code modulated (PCM) signal samples are required to be compatible with either of the A law and .mu.law PCM standards.